1. Field of the Invention
This invention relates generally to power supplies and particularly to electronic circuits for controlling the output of power supplies to provide defined output voltage and current to devices connected to the power supply and to prevent damage to both the power supply and devices connected thereto.
2. Description of the Prior Art
Previous power regulation devices use either a linear or a switching technique for providing electric power having defined voltage and current. Both the linear and the switching regulators have deficiencies that render them unattractive for certain applications.
A switching regulator operates by chopping the unregulated DC supply voltage using a saturated transistor switch and then filtering the chopped voltage. The output voltage is regulated by varying the duty cycle of the transistor. Switching regulators tend to be lightweight, compact and efficient. Switching regulators are limited in transient response by the switching frequency, and are therefore usually slower than linear regulators (milliseconds versus microseconds). In some systems, such as radio frequency circuits, a switching regulator cannot be used because of its high switching noise. Therefore, such systems typically include highly dissipative linear regulators which avoid excessive noise but present problems associated with thermal stresses and heat dissipation.
Compared to switching regulators, linear regulators are simpler and have much lower output noise but are much less efficient at low voltages such as 5 volts, for example. Existing linear regulators using bipolar transistors require a 3 V input-output differential for proper operation. The resulting power dissipation is 31.sub.o volts, where I.sub.o is the current output from the regulator. For a 10 amp output current, which is typical for many radio frequency systems, the minimum power dissipation in the regulator is 10 watts.
U.S. Pat. No. 3,173,078, which issued Mar. 9, 1965 to Farnsworth, is directed to an overload protective power supply that includes an overload sensing curcuit. Farnsworth discloses a power supply having a tunnel diode coupled in a load current path for sensing the load current. A switching transistor is connected between the tunnel diode and a voltage comparator for disconnecting the load from the power supply when the voltage across the tunnel diode indicates an overload condition.
U.S. Pat. No. 3,959,713, which issued May 25, 1976 to Davis et al., is directed to a current limit circuit for limiting the current drain of a series connected load by switching the current limiting circuit from a low impedance state to a high impedance state in response to an overload to prevent excessive power dissipation by the load. Davis et al. dicloses switching circuitry for switching a current limiting transistor from low impedance to high impedance in response to an overload, which limits the current to a predetermined acceptable magnitude. Davis et al. also discloses a thermal resonse circuit that renders the current limiting circuit inoperative when the temperature exceeds a predetermined value. The thermal shutdown circuit includes temperature-sensitive circuitry for switching the current limiting transistor to its high impedance state when the temperature increases to a predetermined value.
U.S. Pat. No. 4,180,768, which issued Dec. 25, 1979 to Ferraro, is directed to an energy limiting foldback circuit for use with a power supply having a power control device in its output. The energy limiting circuit includes a voltage sensor for sensing the voltage between the input and the output of the power control device. A latch places the power control device in a nonconductive state, and a pulse generator periodically places the power control device in a momentary conduction state constrained by the upper current limit of the current that is to be permitted in the output.
U.S. Pat. No. 4,288,831, which issued Sept. 8, 1981 to Dolikian, discloses a switch mode power supply in which a feedback loop varies the duty cycle of a voltage controlled oscillator to control the DC voltaqe output.
U.S. Pat. No. 4,340,851, which issued July 20, 1982 to Nishikawa, is directed to a powerless start circuit for self-biased circuits. A resistive element provides a current path from the supply voltage to the self-biased circuit and a field effect transistor. The transistor responds to the flow of current through the resistor and supplies an initial current to the self-biased circuit. Regenerative feedback causes the circuit to draw a current related to the biasing currents through the current path as the bias currents reach their non-zero operating points. The transistor shuts off the initial current to the self-biased circuit as the nonzero operating point is reached so that the start-up circuit is effectively disconnected from the biasing circuit.
U.S. Pat. No. 4,521,726, which issued June 4, 1985 to Budnick, is directed to control circuitry for a pulse width modulated switching power supply. A pulse generator provides a train of pulses at a predetermined repetition frequency, and a signal source generates a reference voltage. A comparator compares the power supply output to the reference voltage and produces a first state of a switching control signal in response to each trigger pulse from the pulse generator. The comparator produces a second state of the switching control signal when the power supply voltage equals the reference voltage. The first state of the switching signal enables switching transistors in the power supply, and the second state disables the switching transistors. The comparator includes circuitry for reducing the reference voltage when the transistors are disabled to prevent them from being reenabled until the next trigger pulse from the pulse generator changes the state of the control signal.